Removing salts from polyacrylonitrile



Patented Aug. 11, 1953 REMOVING-SALTS FROM POLY-' ACRYLONITRILE George W. Stanton, WalnutCreek, and Theodore B. Lefferdink, Concord, Calif., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Original application May 28, 1951, Serial No. 228,759. Divided and this application October 27, 1952, Serial No. 319,182

3 Claims.

This invention relates to a method for removing metal salts from polymeric films or filaments composed chiefly of acrylonitrile. It is concerned particularly with such a method for minimizing the salt content of fibers of polyacrylonitrile which have been spun from aqueous saline solutions of the polymer. It is taught by Rein in U. S. 2,140,921 and by Kropa' in U. S. 2,356,767 that polyacrylonitrile is soluble in concentrated aqueous solutions of certain highly hydrated metal salts, such as zinc chloride, and that such solutions can be spun to make fibers. Such fibers have been criticised in numerous patents as being weak, brittle and full of voids. Such brittleness or weakness may be due in some cases to the voids resulting from too rapid extraction of the salt during coagulation of the filamentary extruded product, but in others it is due to the presence of some of the salt in the final spun product.

In a group of applications filed concurrently with the original hereof, on May 28, 1951, by one or both of the present inventors and others. Serial Nos. 228,751 to 228,758 inclusive, it is disclosed that improved spun products can be made from polyacrylonitrile when the latter is first dissolved in an aqueous solution of at least 30 per cent, and usually not over 50 per cent, of a solvent salt and a lesser amount, from 5 to 25 per cent of a non-solvent salt, in proportions such that the aggregate amount of the two salts is at least 55 per cent, by Weight. The solvent salts are those whose anions are at least as high in the following table as their cations, and the non-solvent salts are those whose anions are lower in the table than their cations. To be useful, both types of salts employed must be mutually soluble in water.

While fibers spun from solutions in such mixed salts are superior to those made from solutions in single solvent salts, it is desirable for most purposes that the amount of salt or salts left in the'finished fiber be reduced to a minimum. It has been found that the rate of extraction of the last few percent of salts from the fibers by Water is very slow, and often may be considered impractical for use in a continuous fiber spinning process.

It is the principal object of the present invention to provide a practical and rapid method for reducing the salt content of articles such as films or filaments which have been formed from aqueous saline solutions of the polymer of acrylonitrile or of copolymers of at least 85 per cent acrylonitrile with another monoethylenically unsaturated compound.

TABLE Lyotropic series for polyacrylom'trile Cations in Increasing Order of Effectiveness l Anions in Decreasing Order of Effectiveness In accordance with the present invention, which will be described with reference to polyacrylonitrile fibers, the desired salt removal is effected after the fibers have been coagulated and stretched in the desired manner. The fiber forming process may entail coagulation of the polymer solution as it is extruded into water, acid or dilute brine, followed by cold stretching or hot stretching, or both, with following or intervening treatments of the fiber with water or other coagulants to reduce the salt content to a value usually between 2 and 5 per cent. Such an amount of residual salt is not detrimental during a subsequent drying operation, though the presence of over 5 per cent salt usually causes the fibers to fuse when heated to 125 C. or so during drying. A much lower salt content is required when the fibers are to be used in making woven or knit goods which may be worn in contact with sensitive skins. Hence, after the final wet stretching treatment, and before the fibers are dried, they are conducted through an aqueous solution of an agent capable of forming a complex with the metal ion or ions of the salt or salts in the fibers. Such agents include chelating agents and other metal sequestering agents. The preferred agents include the alkali metal salts of ethylene diamine metal ions which may be present in the fibers, including those of zinc, calcium, magnesium, tin, aluminum, antimony, arsenic, bismuth,. and others. The treatment is carried out while the fibers are still wet from a prior step in the process, as it has been found that dried fibers containing salt lose it very slowly to a solution of one of the complex-forming o'r sequestering agents, as they do to plain water.

The following example illustrates the practice of the invention, but is not tobe construed as limiting: H

A solution was prepared consisting of '7 per cent by weight of polyacrylonitrile, having a Staudinger molecular weight'of 76,100,216 per cent zinc chloride, 23.2 per cent calcium chloride and 38.2 per cent water. This solution, having a viscosity of 300 poises, lwas extruded through a IOU-hole spinneret, having openings 0.0025 inch in diameter, into water at C. The socoagulated tow of filaments was cold-stretched in air 1.8 times its initial length, and was hotstretched in wet steam 2.4 times its coldstretched length. The low of stretched fiber was washed for 10 seconds with water. Analysis of the product at various stages in the process to this point showed the following values:

Zinc chloride Calcium chloride Polymer Water One portion of the tow was dried to l per cent moisture content, and had 2.9 per cent zinc chloride and 1.7 per 1 cent calcium chloride. Another portion of the wet tow was soaked in water for one minute, and, when dried, had 0.35 per cent zinc chloride and 0.22 per cent calcium chloride. When, however, the wet tow was led through a 10 per cent solution in water at pH 8 of the tetrasodium salt of ethylene diamine tetraacetic acid for one minute, the subsequently dried fibers had 0.04 per cent zinc chloride and 0.08 per cent calcium chloride. By way of contrast, if thetow was dried before being exposed to the solution of sequestering or chelating agent, soaking'in such a solution for 18 hours still leftover 1 per cent total salt in the redried fibers.

In other tests, when the coagulated and stretched product was immersed in aqueous solutions of metal complexing agents while the product was still wet from a prior step, in the process, the alkali metal salts of ethylene diamine tetraproduct. Thus, when the treated product has been digested with nitric acid, burned to constant weight, and the ash analyzed photometrically for calcium and colorimetrically with dithiazine for zinc, values as low as nil for both elements have been obtained, and total values for the two are seldom found to exceed 0.2 per cent. The complexing agents have been found to be most effective when their solutions are adjusted to a pH value near 8, as their normally alkaline condition, pH 11 or higher,

tends to discolor the fibers.

The invention has been illustrated with respect to the treatment of fibers produced by wet spinning of aqueous saline solutions of poly-: acrylonitrile. It is applicable as well to the removal of salts from such other products as films made by extrusion and coagulation of such:

saline solutions.

This application is a division of our application Serial No. 228,759, filed May 28, 1951.; -i

We claim: T w '1. In a method for making shaped articles by coagulation of an aqueous polyvalent metal salt-containing saline solution of a polymer 0011-. taining at least per cent acrylonitrile inthe polymer molecule, the improvement'which con sists in immersing the article, before it h'asbeen' dried, in an aqueous solution of a Water-soluble alkali metal salt of a polyphosphoric acid as a sequestering agent for'the polyvalent metalions in the salt contained inthe coagulated article, until the salt content is reduced to a negligible value, and then drying the article. 7 g 1 2. The method as claimed'in claim 1, wherein the shaped'article is a wet-spun fiber of polyacrylonitrile. 3. The method as claimed in claim 1, wherein the solution of said alkali metal salt is, employed at a pH of about 8. GEORGE W. STANTON. s

THEODORE B. LEFFERDINK.

No references cited. 

1. IN A METHOD FOR MAKING SHAPED ARTICLES BY COAGULATION OF AN AQUEOUS POLYVALENT METAL SALT-CONTAINING SALINE SOLUTION OF A POLYMER CONTAINING AT LEAST 85 PER CENT ACRYLONITRILE IN THE POLYMER MOLECULE, THE IMPROVEMENT WHICH CONSISTS IN IMMERSING THE ARTICLE, BEFORE IT HAS BEEN DRIED, IN AN AQUEOUS SOLUTION OF A WATER-SOLUBLE ALKALI METAL SALT OF A POLYPHOSPHORIC ACID AS A SEQUESTERING AGENT FOR THE POLYVALENT METAL IONS IN THE SALT CONTAINED IN THE COAGULATED ARTICLE, UNTIL THE SALT CONTENT IS REDUCED TO A NEGLIGIBLE VALUE, AND THEN DRYING THE ARTICLE. 